Really great lyrics and tune and there are many excellent versions throughout the years. There is a nice version on The DP Cow Palace 1974 release. As well as 2 Acoustic versions on the Expanded Reckoning release from 1980. Here is a Birdsong>Cassidy from The Greek 1981 that I have always loved.
I'll second ya, Capt. I've always liked the Cow Palace '74 version--the song's debut in fact. The arrangement is very similar to the studio version from Ace and sounds quite different than the "Cassidy"s of '76 onward.